A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs).
A patterning device may be supported by a mask support structure. In use of the lithographic apparatus, the mask support structure may be driven by a positioning device. The positioning device comprises an actuator having a magnet array and coils. It is desirable for the positioning device to be able to achieve acceleration levels as high as possible. However, due to limited force density (i.e. the ratio of force produced by the actuator to mass of the actuator), the mass of the actuator increases with increasing acceleration levels achieved by the actuator. Furthermore, it is desirable for parasitic forces and torques produced by the actuator to be as low as possible. A parasitic force is a force produced by the actuator that is different from the positioning force that the actuator is intended to provide. A parasitic force can reduce the accuracy of the positioning device.
It is desirable to increase the force density of an actuator for a given level of parasitic forces.